1. Field of the Invention
The present invention relates to a machine and method for ‘additive manufacturing’, fabricating a three-dimensional object by spreading a powdered material on a support stage to form a thin layer of the powdered material and stacking such layers on top of each other.
2. Description of Related Art
In recent years, techniques for fabricating a three-dimensional (3D) object by spreading a powdered material to form a thin layer and stacking such layers on top of each other have attracted lots of attention, and many kinds of additive manufacturing techniques have been developed using different powdered materials and different additive manufacturing procedures (see, for example, JP-A-2010-526694).
FIG. 4 is a schematic cross section of a related art additive manufacturing machine, 100. The additive manufacturing machine 100 spreads a metal powder M1 tightly on the top surface of a powder support stage 104 using a powder feeder 107 to form one layer. Then, only a two-dimensional structural portion of the metal powder M1 which has been spread tightly on the stage 104 and which corresponds to one cross section of a three-dimensional object P1 to be created is melted with an electron beam. The three-dimensional object is built by stacking such layers of the metal powder M1 on top of each other in the direction of height (in the Z-direction).
As shown in FIG. 4, the additive manufacturing machine 100 has an electron gun 108 mounted in the top of a vacuum vessel 102. A cylindrical additive manufacturing platform 103 is mounted inside the vacuum vessel 102. An electron beam controller 111 for controlling the electron gun 108 is connected with the electron gun 108. The platform 103 is centrally provided with a pit 103a. A drive mechanism 105 by which the stage 104 is movably supported is mounted below the pit 103a. The support stage 104 has a shaft portion 104d to which the drive mechanism 105 is coupled to drive the stage 104 in the vertical direction. The interior of the vacuum vessel 102 is maintained at a vacuum.
The support stage 104 is placed by the drive mechanism 105 at a position that is lower than the top surface of the additive manufacturing platform 103 by a distance of ΔZ. The metal powder M1 is spread tightly up to a thickness equal to the distance ΔZ on the stage 104 by the powder feeder 107.
A previously prepared, designed three-dimensional model is sliced into multiple two-dimensional shapes which are spaced from each other at intervals of ΔZ. In conformity with one of the two-dimensional shapes, an electron beam L1 is directed from the electron gun 108 at the metal powder M1. The region of the metal powder M1 conforming to the two-dimensional shape is molten by the electron beam L1 emitted from the electron gun 108. When a given time according to the material elapses, the molten metal powder M1 solidifies. After one layer of metal powder M1 is melted and solidified, the support stage 104 is lowered an incremental distance equal to ΔZ by the drive mechanism 105. Then, an amount of the metal powder M1 for achieving a thickness of ΔZ is spread tightly on the lower layer formed immediately previously. The region of the metal powder M1 corresponding to the two-dimensional shape corresponding to this layer is illuminated with the electron beam L1 to melt and solidify the metal powder M1. This series of steps is repeated to stack layers of the melted and solidified metal powder M1 on top of each other, thus creating a three-dimensional object.
FIG. 3A schematically shows the state of electrons when the metal powder M1 is illuminated with the electron beam L1. However, with the additive manufacturing machine 100 shown in FIG. 4, when the electron beam L1 impinges substantially perpendicularly on the sample surface, most of the impinging electrons penetrate into the sample surface and diffuse into the sample layer located under the metal powder M1 spread tightly as shown in FIG. 3A. Some electrons stay in the metal powder M1. Where the metal powder M1 is an insulator, the metal powder M1 is negatively charged by the electron beam L1. Even where the metal powder M1 is a conductor, if the grains of the powder each have a small ground contact area, are spherical in shape, and make a point contact with each other, then the powder is negatively charged in the same way as for an insulator, because the amount of current of the electron beam L1 is large. Therefore, as shown in FIG. 5, the negatively charged grains of the metal powder M1 repel each other. This creates the danger that the powder M1 will be scattered.
In order to prevent the metal powder M1 from being negatively charged and scattering, it is conceivable to mount a gas introduction device 112 on the vacuum vessel 102 to introduce an inert gas F1 for neutralizing the charged metal powder M1 as shown in FIG. 4. When the inert gas F1 is introduced, however, the energy of the electron beam L1 emitted from the electron gun 108 is scattered and lost, thus presenting the problem that the metal powder M1 is not melted. Another problem is that the inert gas F1 adheres to the electron gun 108, shortening the lifetime of the gun 108.